How to change the driver's Expression to "var * 3 + var * 2"?
Change driver type  to scripted expression, (from sum variables) call your variables say sk1 and sk2 and make the expression

3 * sk1 + 2 * sk2 


or 

sum(3 * sk1, 2 * sk2)


Currently you are typing an expression var * 3  into the name field of the variable hence the error.

To do this with sum variables driver type would need to add the sk1 variable 3 times and the sk2 variable twice. (all with unique names)